961 N-Repeated Element in Size 2N Array

#include <iostream>
#include <vector>
 
int main() {
    std::vector<int> v = {0, 1, 2, 3, 4, 5};
 
    for (const int& i : v) // access by const reference
        std::cout << i << ' ';
    std::cout << '\n';
 
    for (auto i : v) // access by value, the type of i is int
        std::cout << i << ' ';
    std::cout << '\n';
 
    for (auto&& i : v) // access by forwarding reference, the type of i is int&
        std::cout << i << ' ';
    std::cout << '\n';
 
    const auto& cv = v;
 
    for (auto&& i : cv) // access by f-d reference, the type of i is const int&
        std::cout << i << ' ';
    std::cout << '\n';
 
    for (int n : {0, 1, 2, 3, 4, 5}) // the initializer may be a braced-init-list
        std::cout << n << ' ';
    std::cout << '\n';
 
    int a[] = {0, 1, 2, 3, 4, 5};
    for (int n : a) // the initializer may be an array
        std::cout << n << ' ';
    std::cout << '\n';
 
    for (int n : a)  
        std::cout << 1 << ' '; // the loop variable need not be used
    std::cout << '\n';
 
}

#include <iostream>
#include <set>
#include <vector>

using namespace std;

int repeatedNTimes(vector<int> &A)
{
    set<int> m;
    for (vector<int>::iterator it = A.begin(); it != A.end(); it++) {
        if (m.find(*it) != m.end()) {
            return *it;
        }
        else {
            m.insert(*it);
        }
    }
}

class Solution {
public:
    int repeatedNTimes(vector<int>& A) {
        unordered_set<int> seen;
        for (int a: A) {
            if (seen.count(a))
                return a;
            seen.insert(a);
        }
        return 0;
    }
};